Global Navigation Satellite Systems (GNSS) include the Global Positioning System (GPS), the Glonass system, the proposed Galileo system, the proposed Compass system, and others. Each GPS satellite transmits continuously using two radio frequencies in the L-band, referred to as L1 and L2, at respective frequencies of 1575.42 MHz and 1227.60 MHz. Two signals are transmitted on L1, one for civil users and the other for users authorized by the United States Department of Defense (DoD). One signal is transmitted on L2, intended only for DoD-authorized users. Each GPS signal has a carrier at the L1 and L2 frequency, a pseudo-random noise (PRN) code, and satellite navigation data. Two different PRN codes are transmitted by each satellite: a coarse acquisition code and a precision (P/Y) code which is encrypted for DoD-authorized users. Each C/A code is a unique sequence of 1023 bits, which is repeated each millisecond. New GPS satellites are able to broadcast on 3 frequencies. Other GNSS systems likewise have satellites which transmit multiple signals on multiple carrier frequencies.
FIG. 1 schematically illustrates a typical prior-art scenario to determine the position of a mobile receiver (rover). Rover 105 receives GNSS signals from one or more satellites in view, such as satellites 110, 115, 120, 125 and 130 shown. The signals pass through the earth's atmosphere 160, the upper portion is called the ionosphere, while the lower portion of the atmosphere is referred to as the troposphere. The multi-frequency GNSS PRN code and carrier phase signals are simultaneously tracked by the rover receiver and by one or more GNSS reference receivers 135 and 140. The ionosphere causes a dispersive effect whereby the code is delayed, while the carrier phase is advanced. The troposphere delays the signals with the magnitude of the effect dependent on the prevailing atmospheric temperature, pressure, relative humidity and precipitable water vapor content.
Each satellite broadcasts a prediction of its expected orbital trajectory in a navigation message. The navigation message also includes a prediction of the expected satellite clock behavior. The satellite clock, orbit and atmospheric errors can be considered as causing an apparent shift in the satellite locations 110->170, 115->175, 120->180, 125-185, 130-190, as depicted in FIG. 1.
Prior-art network GNSS processing techniques such as described in U.S. Provisional Application for Patent No. 61/277,184 filed 19 Sep. 2009, enable satellite and atmospheric errors to be estimated by first tracking the satellite signals at a network of reference stations, spatially distributed globally and/or regionally. The satellite orbit/clock and atmospheric errors are estimated in a network processor such as 145, in FIG. 1. The satellite correction data is then encoded and transmitted via antenna 150, for later reception and use by one or more rovers 105
In prior-art rover processing techniques such as described in International Patent Application PCT/US2010/02562 filed 19 Sep. 2010, the rover GNSS data is combined with the GNSS network correction data at a plurality of epochs in order to estimate the rover antenna position plus nuisance parameters such as a set of multi-frequency (carrier phase) ambiguities and tropospheric biases
FIG. 2 is a block diagram of a typical integrated receiver system 200 with GNSS rover 105 and communications antenna 202. Receiver system 200 can serve as rover 105 or as a reference station. Receiver system 200 includes a GNSS receiver 205, a computer system 210 and one or more communications links 215. Computer system 210 includes one or more processor(s) 220, one or more data storage elements 225, program code 230 for controlling the processor(s) 220, and user input/output devices 235 which may include one or more output devices 240 such as a display or speaker or printer and one or more devices 245 for receiving user input such as a keyboard or touch pad or mouse or microphone.
FIG. 3 illustrates the horizontal position accuracy convergence over time of a GNSS rover position solution based on global GNSS network correction data and precise satellite orbit and clock data. The x-axis represents time in minutes, while the y-axis represents the horizontal position accuracy in centimeters. The x-y axes are denoted 405. The trace of horizontal position accuracy is denoted 410. Many high precision applications require for example 2.5 cm (1 inch) horizontal position accuracy; this threshold is indicated by the dashed horizontal line 415. The convergence time (420) needed to achieve the position threshold is around 18 minutes in this example. A tracking interruption at the rover receiver occurs around time 40 minutes (denoted 425). The tracking interruption leads to a reconvergence period 430.
Based on GPS satellites alone, it is common to have convergence times of 10-30 minutes to achieve a horizontal position accuracy of 2.5 cm. Many GNSS applications need cm-level accuracy and therefore the convergence time hinders the usefulness of the system. It is common for satellite tracking to be interrupted from time-to-time on one or more satellites at the rover, particularly when the rover is moving. If the number of tracked satellites drops below 4, the solution is converged again as shown in FIG. 3.
In prior-art processing techniques, Geng, J, et al, 2010, Rapid re-convergences to ambiguity-fixed solutions in precise point positioning, Journal of Geodesy, Vol 84, pp 705-714, a technique is described for improving the re-convergence of precise point positioning following tracking interruptions. Specifically, the wide-lane ambiguities are estimated first with the aid of ionospheric-free code measurements. Next a linear time-window-based prediction of the ionospheric bias on each cycle slipped satellite is made. The predicted ionospheric bias is used to limit the search space of narrow-lane phase ambiguities. The reported results from ibid, show re-convergence times of 5 seconds in most tests. Few details are provided on the filtering scheme used for the PPP solution.
In prior-art processing techniques, Banville, S, and Langley, R. B., 2010, Instantaneous Cycle-Slip Correction for Real-Time PPP Applications, NAVIGATION, Journal of the US Institute of Navigation, Vol 57 No 4, Winter, pp 325-334, describe a way of repairing cycle slips in Precise Point Positioning (PPP) applications based on time-differenced phase measurements. First an attempt is made to fix the time-differenced wide-lane carrier phase ambiguities following an interruption to satellite tracking. Next the known wide-lane ambiguities are used in conjunction with the assumed time-wise change in ionospheric bias to fix the L1 and L2 on each satellite during the tracking interruption.
Precise Point Positioning (PPP) techniques involve careful modeling of various error sources affecting satellite measurements. Real-time rover processors are often limited in terms of size, weight and power and therefore careful consideration must be given to efficient data processing techniques that minimize compute power. The prior-art methods for cycle slip repair in PPP applications do not mention the use of distributed filtering for the underlying state parameter estimation.
Improved GNSS processing methods and apparatus are desired, especially to achieve faster and more efficient convergence to a solution, improved accuracy and/or greater availability.